Abrasion resistance is required for an automobile component, a power transmission component for industrial machines, such as a chain components and a gear wheel, and a cutting tool, such as a rim saw and a band saw, used for cutting wood, grass or the like. In general, the abrasion resistance of a steel material is enhanced by increasing the hardness thereof. Accordingly, the component where the abrasion resistance is important is generally produced with a steel material that has been tempered to have higher hardness through tempering performed at a low temperature after quenching, and a steel material having a large content of an alloy element, such as carbon. Accordingly, there is a close relationship between the hardness and the abrasion resistance of the steel material, and as a measure for imparting abrasion resistance to a steel material, a measure for enhancing the hardness thereof has been ordinarily employed.
For example, PTLs 1 to 3 describe that in a steel having a C content of approximately 0.2% or less, the content of the alloy element is set to a higher value to enhance the hardness through solid solution strengthening, precipitation strengthening and the like. However, associated with the increasing demand level of abrasion resistance in recent years, the sufficiently satisfactory abrasion resistance often cannot be obtained only by increasing the hardness. Furthermore, the increase of the content of the alloy element as in PTLs 1 to 3 lowers the productivity and the workability of the material as a result, which causes a problem of increase in production cost.
On the other hand, it is important to prevent a power transmission component and a cutting tool from being broken in use, from the standpoint of safety. For preventing the breakage, it is necessary to ensure sufficiently the toughness of the steel material used for the component. For enhancing the toughness of the steel material, in general, the suppression of the tempered hardness is said to be effective. However, the abrasion resistance is generally lowered simultaneously along with the suppression of the tempered hardness. That is, the abrasion resistance and the toughness of the steel material are in a trade-off relationship.
The applicant has made various investigations on a technique capable of achieving both the abrasion resistance and the toughness simultaneously, and describes the practical measure in PTL 4. The measure is to enhance the abrasion resistance by utilizing dispersion of a Nb-containing carbide without the use of a Ti based carbide, which is a factor decreasing the toughness. On casting a Nb-containing steel, the period of time for retaining the cast material at a high temperature is made sufficiently long, so as to deposit the sufficient amount of the Nb-containing carbide excessively, and a part of the Nb-containing carbide re-dissolves into a solid solution through the subsequent heat treatment, thereby controlling the precipitated amount of the Nb-containing carbide. According to the procedure, the resistance to abrasive abrasion can be enhanced while maintaining the toughness, which is effective for the enhancement of the lifetime of the high strength mechanical member. The abrasive wear is such a mode of wear that the surface of the material is scraped off with the unevenness on the frictional surface of the counter material or with the foreign matters intervening between the frictional surfaces.